In the case of what are referred to as multi-point injection (MPI) engines with one or more injection valves per cylinder or else in the case of direct injection engines, an increase in the leakage of the injection valves may occur due to soiling of the injection valves, due to a production tolerance and/or due to wear in the region of the sealing seat of the injection valves. Soiling of the injection valves can occur, for example, as a result of operation with low-additive fuels (too few cleaning additives in the fuel) over a relatively long period of time. The resulting deposits on the sealing seat can lead to a reduction in the through-flow quantity and to an increase in the leakage of the injection valves.
Reducing the through-flow of the injection valves is typically detected by a lambda controller and compensated. However, a leakage of fuel in the shut-down state of the engine or of the internal combustion engine causes fuel to be present in the intake manifold (or in the cylinder) in the form of gaseous fuel vapor or liquid fuel.
In the case of a starting process of an internal combustion engine, an engine controller measures the fuel quantity which is necessary for starting the engine in a manner which is as low in friction, and therefore smooth, as possible, as a function of various characteristic variables such as the air quantity, engine temperature, ambient temperature, etc. In this context, over-enrichment of the fuel/air mixture can be caused by an increased injection valve leakage during the starting process. Depending on the degree of leakage, this over-enrichment can go as far as giving rise to a fuel/air mixture which cannot be ignited. This in turn causes a significantly delayed start by virtue of the fact that, depending on the parameters of the engine swept volume and intake manifold volume, a specific number of engine revolutions is required until the engine actually starts. Apart from a resulting worsening of the emissions of pollutants, the driver therefore also experiences a significant loss of comfort, in particular in the case of a relatively long shut-down time.
In order to improve, inter alia, the starting behavior of an internal combustion engine, methods for adapting the fuel quality are known which evaluate, for example, the rotational speed gradient (=change in the engine rotational speed over time) when the engine starts. These methods bring about a correction in the fuel quantity or fuel mass to be injected for the entire starting process of the engine with the objective of adapting the fuel quantity or fuel mass which is present in a gaseous form in the cylinder of the engine, in order in all cases to obtain a mixture which can be ignited and to set a predefined revving up of the rotational speed until the idling rotational speed is reached. In this context, depending on the fuel quality, global corrections with an increasing or reducing effect are possible for the starting process. Methods for adapting the fuel quality are applied as a matter of priority after tank detection and when the engine is warm. An injection valve leak which may, under certain circumstances, be present causes, if it is at all detected by the adaptation, an incorrect correction value of the adaptation of the fuel quality.